Programming The Watchdog Timer - Aaeon PCM-5335 Manual

A.1 Programming the watchdog timer
The default configuration of the timer is enabled with system reset.
To enable the watchdog timer, you must make a program which
writes 1 to I/O port address 443 (hex) at regular intervals. The first
time your program reads the port, it enables the watchdog timer. After
that, your program must write 1 to port 443 at time intervals of less
than 1.6 seconds. Otherwise, the watchdog timer will activate and
reset the CPU. When you want to disable the watchdog timer, your
program should write 0 to port 443.
If CPU processing comes to a standstill because of EMI or a software
problem, your program's signals to I/O port address 443 of the timer
will be interrupted. The timer will then automatically reset the CPU,
and data processing will continue normally.
You must make your program so that it writes 1 to I/O port 443 at an
interval shorter than the timer's preset interval. The timer's intervals
have a tolerance of ± 30%, so you should program an instruction that
will refresh the timer about every second.
5 8 PCM-5335 User's Manual